Population dynamics and environmental variation across the geographic range of a California annual plant

The aim of our study is to test theoretical models on environmental and demographic limits to range expansion. Data indicate that abrupt changes in precipitation and soil bedrock at one range edge lead to greater plant water stress compared to the center of the range. Two biotic factors, pollinators and herbivores, reduce and increase, respectively, reproductive success at the edge vs. the center. Demographic tests of the "abundance center" hypothesis that central populations are larger and more reproductively successful than range edge populations are not supported by our data. Tests of greater demographic stochastic and extinction risk in edge populations will require additional years of study. However, one important mitigating factor on population extinction is the persistence of viable seeds in the soil. Preliminary experimental studies of seed viability in the field do not indicate higher long-term viability of range edge compared to center derived seed, in spite of genetic evidence of higher seed dormancy in edge populations in the greenhouse. We discuss the implications of our empirical findings to theory on range expansion.